1. Field of the Invention
The present invention relates to solid-state imaging devices and more particularly, to solid-state imaging devices of one- or two-dimensions using a charge transfer device such as a charge coupled device (CCD).
2. Description of the Related Art
Compared to an image pickup tube which has been popularly used for imaging, a solid-state imaging device has many advantages such as compactness, light-weight, supreme endurance, low-power consumption, and little residual image, little sticking.
In recent years, solid-state imaging devices have already become more popular then image pickup tubes in private use, such as in movie cameras where the applicable image size is relatively small. Also in business use, such as a television camera where the applicable image size is relatively large, solid-state imaging devices are close to replacing image pickup tubes.
FIG. 6 schematically shows a solid-state imaging device of the interline transfer type.
In FIG. 6, a plurality of photoelectric conversion sections 31 are arranged in a matrix array. Each of the photoelectric conversion sections 31 receives incident light to generate and store an electrical signal charge according to the intensity of the incident light thus received.
A plurality of vertical charge transfer sections 32 are arranged along the respective columns of the matrix. Each of the vertical charge transfer sections 32 reads out the signal charges stored in the photoelectric conversion sections 31 of each column to transfer vertically the signal charges thus read to a horizontal transfer section 33.
The horizontal transfer section 33 transfers the signal charges from the vertical charge transfer sections 32 horizontally to a output section 34.
The output section 34 converts the signal charges from the horizontal transfer section 33 to a voltage be output.
FIG. 1 shows a first example of the conventional solid-state imaging device with the above structure shown in FIG. 6, in which a cross-sectional view along the line 1--1' is drawn.
A p-well layer 402 is formed on an n-semiconductor substrate 401. In the photoelectric conversion section 31, there are formed a p.sup.+ -photoelectric conversion region 404 and a p.sup.+ -channel stop region 406 for isolation, both of which are connected with each other, in the surface area of the p-well layer 402. An n-photoelectric conversion region 403 is formed under the p.sup.+ -photoelectric conversion region 404. The p.sup.+ - and n-photoelectric conversion regions 404 and 403 form a photoelectric conversion member or photodiode.
In the charge transfer section 32, there is formed an n-charge transfer region 405 in the surface area of the p-well layer 402. One end (left-hand end in FIG. 1) of the n-charge transfer region 405 is apart from the p.sup.+ -photoelectric conversion region 404 and the other end (right-hand end in FIG. 1) is connected to a p.sup.+ -channel stop region 406 in an adjacent one of the photoelectric conversion sections 31.
An insulation film 408 of silicon dioxide (SiO.sub.2) is formed on the surface of the p-well layer 402 to cover both of the photoelectric conversion section 31 and the charge transfer section 32.
A transfer electrode 407 made of polysilicon is formed within the SiO.sub.2 insulation film 408 in the charge transfer section 32. Between the transfer electrode 407 and the surface of the p-well layer 402, there is a part of the SiO.sub.2 insulation film 408. One end (left-hand end in FIG. 1) of the electrode 407 is extending to the end (right-hand end in FIG. 1) of the p.sup.+ -photoelectric conversion region 404 and the other end (left-hand end in FIG. 1) is extending to the end (left-hand end in FIG. 1) of the n-photoelectric conversion region 403 in the adjacent one of the photoelectric conversion sections 31.
In the charge transfer section 32, a light shielding film 410 of metal is formed on the SiO.sub.2 film 408 to prevent the incident light from entering the p-well layer 402 in the charge transfer section 32. The light shielding film 410 is not formed in the photoelectric converting section 31.
A protection film 411 of SiO.sub.2 is formed to cover the light shielding film 410 and the SiO.sub.2 insulation film 408 exposed from the film 410.
With the solid-state imaging device described above, when the incident light enters the p.sup.+ - and n-photoelectric conversion regions 404 and 403 forming the photodiode through the passivation film 411 and the insulation film 408, the light is converted to generate a signal charge and the signal charge thus generated is temporarily stored therein.
Under application of a driving voltage into the transfer electrode 407, the signal charge in the photodiode is moved through a field shift gate 412 to the charge transfer section 32, and then transferred vertically toward the horizontal transfer section 33.
All of the photoelectric converting sections 31 belonging to the respective columns operate in the same way as described above in response to the driving voltage. As a result, the signal charges stored in the photoelectric converting sections 31 are successively transferred through the respective vertical transfer sections 32 to the horizontal transfer section 33. The signal charges are then transferred to the output section 34 by the horizontal transfer section 33 and are converted to an output voltage signal in the output section 34 to be outputted therefrom.
With the solid-state imaging device described above, there arises a phenomenon called "smear" that worsen the image quality, which is never seen in the image pickup tube. The "smear" phenomenon occurs due to the following cause:
When picking up a subject of high-level luminance, the incident light entering the photoelectric converting section reaches also the neighborhood of the vertical transfer section 32 due to diffraction of the incident light. The light entered the neighborhood of the section 32 excites some electric charges therein and the charges leak into the vertical transfer section 33. As a result, a false or error signal is generated to be outputted from the output section 34, providing deterioration in image quality.
To restrain the smear phenomenon, solid-state imaging devices as shown in FIGS. 2 and 3 have been developed, which are disclosed in the paper by Y. ISHIHARA ET AL, entitled "A HIGH PHOTOSENSITIVITY IL-CCD IMAGE SENSOR WITH MONOLITHIC RESIN LENS ARRAY", IEDM, 83 PP497-500 (1983) and in the JAPANESE UN-EXAMINED PATENT PUBLICATION NO. 61-49467, respectively.
The second example of the conventional solid-state imaging devices in FIG. 2 is the same in configuration as that shown in FIG. 1 excepting that a flattening film 520 is formed on the light shielding film 410 and the insulation film 408 instead of the protection film 411 and that an array of microlenses 521 are arranged on the flattening film 520 at positions corresponding to the respective photoelectric converting sections 31. The microlenses 521 are made of photosensitive polymer resin.
In the device shown in FIG. 2, the incident light is collected into the centers of the respective photoelectric converting sections 31 by the array of the microlenses 521 to prevent the incident light from entering the neighborhood of the vertical transfer section 32.
The third example of the conventional solid-state imaging devices in FIG. 3 is the same in configuration as that shown in FIG. 1 except that a silicon nitride (Si.sub.3 N.sub.4) film 620 is formed on the light shielding film 410 and the insulation film 408 instead of the protection film 411 and that an SiO.sub.2 protection film 611 is formed on the Si.sub.3 N.sub.4 film 620.
In the device shown in FIG. 3, the incident light is collected into the centers of the respective photoelectric converting sections 31 by the Si.sub.3 N.sub.4 film 620 due to the refractive index difference between the SiO.sub.2 film 408 and the Si.sub.3 N.sub.4 film 620.
The conventional solid-state imaging device shown in FIGS. 2 and 3 have the following problems:
With the device in FIG. 2, when the microlens 521 are optimized in curvature and height for the incident light with the given or designed angle of incidence, the incident light without the designed angle of incidence is collected to a point or area shifted from the designed collection point or area. As a result, the obtainable effect of restraining the smear phenomenon is not sufficient.
With the device in FIG. 3, when the Si.sub.3 N.sub.4 film 620 is optimized in refractive index and thickness for the incident light with the given or designed angle of incidence, the obtainable effect of restraining the smear phenomenon is not sufficient due to the same reason as that in FIG. 2.
To solve the above problems, with the type of the conventional solid-state imaging device shown in FIG. 1, the distance d.sub.1 ' between the p.sup.+ -photoelectric conversion region 404 and the SiO.sub.2 protection film 411 is set smaller to restrain the diffraction effect of the incident light, providing reduced quantity of the incident light into the neighborhood of the charge transfer section 32. This is disclosed in the paper by T. TERAHASHI ET AL, entitled "A FLAME INTERLINE TRANSFER CCD IMAGE SENSOR FOR HDTV CAMERA SYSTEM", IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL.ED-34, PP1052-1056 (1987).
With the conventional solid-state imaging device shown in FIG. 1, as the distance d.sub.1 ' between the p.sup.+ -photoelectric conversion region 404 and the SiO.sub.2 protection film 411 is set smaller, the distance d.sub.2 ' between the transfer electrode 407 and the light shielding film 410 decreases. Therefore, there arises a problem that the distance d.sub.1 ' is set excessively small, the dielectric breakdown strength between the transfer electrode 407 and the light shielding film 410 decreases remarkably.
As a result, the distance d.sub.1 ' cannot be set so small that the smear phenomenon is sufficiently restrained.
For example, in the case that the transfer electrode 407 is made of a polysilicon film doped with phosphorus and produced by a thermal oxidation process, the insulation film 408 is made of an SiO.sub.2 film produced by a thermal oxidation process, and the light shielding film 410 is made of an aluminum (Al) film, the SiO.sub.2 insulating film 408 is required to be approximately 200 nm in thickness.